Voluntary sector bodies increasingly have to evaluate their impact on outcomes and to try to work out how cost-effective they are – it is becoming harder for them to obtain funding otherwise. The government, in particular, seems to hope that this could be a pathway to many charities funding themselves through social impact bonds. But although some evaluation work is useful, our work together shows that limitations need to be recognised.

Family Rights Group (FRG) is a small NGO that through an advice line supports parents and families whose children are involved with or need social care services, or are at risk of going into care. The charity worked with FTI Consulting to try to assess its outcomes as part of the tendering process to run a helpline.

Working out the cost per intervention is pretty simple. FRG targets the most difficult cases, so its calls to its advice line are longer and its advisers more qualified and experienced than some other groups.

The value for money case involves considering whether the additional outcomes from such specialist support outweighs those extra costs. And that is tough to measure.

FRG has many aims – including improved family functioning. But when trying to prove value for money to the government, it is the public costs avoided that is important. For FRG, the key measure is how many children did not go into care who otherwise would have.

This is a major ask of any voluntary sector body. Ideally, control groups are needed – but that would almost involve deliberately refusing to help every other call and seeing whether there was a difference in outcomes. Such controlled experiments are virtually impossible in this area – and questionable ethically.

Instead, FRG had good data on the characteristics of people contacting the charity and the issues they were facing. This gave some feel of the probability of children who might go into care without the charity's support. FRG had also undertaken some follow-up surveys (a difficult task as this client group often moves around, changes contact details regularly and is not keen to talk).

Would all of those children involved with FRG have gone into care without the charity's support? Would the families have got advice from elsewhere, and what would that advice have achieved? In these areas, intelligent assumptions had sometimes to be made in conjunction with the advice of experts.

The savings made when preventing a child going into care are easy to calculate, but problematic. The literature does not take into account the public service savings made by improved health, let alone the higher future earnings that children who have not been in care command. We still had to make some assumptions about how long a child would have been in care without the intervention.

Putting all this together, we found that FRG was very good value for money – with a conservative estimate that for every £1 spent, the taxpayer saved £11.

FRG was successful in its helpline tender. But it is hard to say whether similar assumptions and methodologies were used by other NGOs.

It was useful that FRG had to think about what its outcomes were, how it achieved them and how to collect better data. But there is a limit to what this approach can achieve, and it can force NGOs to focus only on what is measurable. The trend for outcome-based approaches, contracts and finance might in fact help destroy good outcomes.

• Cathy Ashley is chief executive of Family Rights Group; Dan Corry is a director at FTI Consulting.